未捕获的TypeError：无法读取未定义的属性'0' - OpenWeatherMap API

Question

我一直得到这个未捕获的TypeError：无法读取未定义的属性'0'

代码完美无缺，控制台或其他任何东西都没有错误，但是一切都会出现这个错误，然后就会消失。有谁知道为什么？

var temp, loc, icon, hum, wind, dir;

var APPID = "56a4fe4f1240df2a9fe267b687a5d191";

function update (weather) {
    temp.innerHTML += weather.temp;
    loc.innerHTML += weather.loc;
    hum.innerHTML += weather.hum;
    wind.innerHTML += weather.wind;
    dir.innerHTML += weather.dir;
    sUp.innerHTML += weather.sUp;
    sDown.innerHTML += weather.sDown

    icon.src = "//openweathermap.org/img/w/" + weather.icon + ".png";
}

window.onload = function () {
    temp = document.getElementById ("t");
    loc = document.getElementById ("l");
    icon = document.getElementById ("i");
    hum = document.getElementById ("h");
    wind = document.getElementById ("w");
    dir = document.getElementById ("d");
    sUp = document.getElementById ("sU");
    sDown = document.getElementById ("sD");

    if (navigator.geolocation) {
        var showPos = function (pos) {
            updateLoc (pos.coords.latitude, pos.coords.longitude);
        }
        navigator.geolocation.getCurrentPosition (showPos);
    } else {
    var zip = window.prompt("Your location could not be found, please enter your post/zip code.");
    updateByZip(zip);
    }
}

function updateLoc (lat, long) {
    var url = "http://api.openweathermap.org/data/2.5/weather?" + 
        "lat=" + lat + 
        "&lon=" + long + 
        "&APPID=" + APPID;
    sendRequest (url);
}
function updateByZip(zip){
    var url = "http://api.openweathermap.org/data/2.5/weather?" +
    "zip=" + zip +
    "&APPID=" + APPID;
    sendRequest(url);
}

function sendRequest (url) {
    var xmlhttp = new XMLHttpRequest ();
    xmlhttp.onreadystatechange = function () {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
            var data = JSON.parse (xmlhttp.responseText);

            var weather = {};
            weather.icon = data.weather[0].icon;
            weather.dir = direction (data.wind.deg);
            weather.wind = mph (data.wind.speed) + "mph";
            weather.temp = K2C (data.main.temp) + "°C";
            weather.loc = data.name;
            weather.hum = data.main.humidity + "%";
            weather.sUp = formatDate (data.sys.sunrise);
            weather.sDown = formatDate (data.sys.sunset);

            update (weather);
        }
    };
    xmlhttp.open ("GET", url, true);
    xmlhttp.send ();
}

function K2F (k) {
    return Math.round(k * (9 / 5) - 459.67);
}
function K2C (k) {
    return Math.round (k - 273.15);
}
function mph (ms) {
    return Math.round (ms / 0.44704);
}
function kmph (ms) {
    return Math.round (ms * 3.6);
}
function formatDate (t) {
    var date = new Date (t * 1000);
    var h = date.getHours ();
    var m = "0" + date.getMinutes ();
    var s = "0" + date.getSeconds ();
    return h + ":" + m.substr(-2) + ":" + s.substr(-2);
}
function direction (deg) {
    var range = 360 / 16;
    var low = 360 - (range / 2);
    var high = (low + range) % 360;
    var angles = ["N", "NNE", "NE", "ENE", "E", "ESE", "SE", "SSE", "S", "SSW", "SW", "WSW", "W", "WNW", "NW", "NNW"];
    for (var i in angles) {
        if (deg >= low && deg < high) {
            return angles[i]
        }
        low = (low + range) % 360;
        high = (high + range) % 360;
    }
    return "N";
}